The present invention relates to an improved power steering apparatus for a vehicle and, more particularly, to a hydraulically powered steering apparatus having an electrically powered backup.
A typical power steering apparatus for a vehicle is hydraulically operated. The apparatus includes a pump that is powered by the vehicle engine. The pump circulates hydraulic fluid though a hydraulic system containing a fluid reservoir. The pressurized hydraulic fluid is utilized by the power steering apparatus to aid the vehicle driver in turning the steerable wheels of the vehicle.
U.S. Pat. No. 4,942,803 discloses a hydraulic power steering gear. The power steering gear has a housing defining a fluid chamber, and a piston in the fluid chamber. The piston contains rack teeth that mesh with the teeth of a sector gear. The sector gear is fixed on an output shaft that transmits its movement to the respective wheels. The power steering gear is connected to a steering wheel of the vehicle by a shaft. When the vehicle driver turns the steering wheel, the shaft rotates a valve core part of the steering gear with respect to a valve sleeve part of the steering gear. This action causes hydraulic pressure to move the piston.
If the hydraulic power steering apparatus becomes inoperative, a driver must manually be able to steer the vehicle. To allow this manual steering when the hydraulic system is inoperative, the valve core part, the valve sleeve part, and the piston become mechanically linked. Thus, turning the steering wheel causes the valve core part and the valve sleeve part to turn and the piston to move, resulting in manual steering of the vehicle.
Although the manual steering of the vehicle is effective, this steering is difficult, particularly for large vehicles. As a result, there is a need for a system to assist with the steering of a vehicle should the hydraulic system become inoperative.
The present invention is an apparatus for turning steerable wheels of a vehicle. The apparatus includes a drive mechanism, a first motor, a second motor, a sensor for sensing that the first motor is inoperative, and a torque sensor. The drive mechanism is responsive to the turning of the steering wheel of the vehicle, and motion of the drive mechanism turns the steerable wheels. The first motor, for inputting motion to the drive mechanism, is hydraulically powered. The second motor, for inputting motion to the drive mechanism, is electrically powered. The sensor for sensing that the first motor is inoperative produces an enabling signal and communicates the enabling signal to the second motor to enable the operation of the second motor. The torque sensor responds to the turning of the steering wheel and controls the second motor when the second motor is enabled. The first motor is a hydraulically powered. The second motor is electrically powered and preferably includes a motor drive circuit for controlling operation of the second motor. If the driver-applied torque to the steering wheel reaches a predetermined level, the torque sensor communicates a torque signal to the motor drive circuit. The motor drive circuit controls the second motor in accordance with the torque signal.